The present invention relates to a plate for printing and a method of using it, which are employed in patterning of a paste, filling a paste in a through hole and the like to prepare two-sided or multilayered wiring boards for use in a variety of electronic equipment.
In recent years, as a reduction in size and an increase in assembly density are in progress in electronic equipment, an increasingly multilayered structure has been required of circuit boards not only in the industrial electronics area but also in the consumer electronics area. With such circuit boards, it has become absolutely necessary to develop a new inner via-hole joining method for connecting between the circuit patterns disposed on a plurality of layers and also a new and highly reliable structure. In this respect, a new technology is disclosed to realize a high density circuit board manufacturing method of new construction featuring an inner via-hole connection performed by the use of a conductive paste. (Refer to the Japanese Patent Application Unexamined Publication No. H6-268345.) A description is given to this circuit board manufacturing method in the following:
First, a conventional manufacturing method of two-sided circuit boards is described.
FIG. 8(a) to FIG. 8(f) show cross-sectional views of the steps constituting a conventional manufacturing method of two-sided circuit boards. FIG. 9 is a perspective view of a conventional plate framework with an opening provided and a mask attached thereto. FIG. 10 is a cross-sectional view of the foregoing conventional plate framework with an opening provided and a mask attached thereto. FIG. 11(a) to FIG. 11(g) show cross-sectional views of the paste filling steps performed according to a squeegeeing method.
In FIG. 8, the reference numeral 21 is a prereg sheet measuring 300 mm wide, 500 mm long and 150 xcexcm thick and using a board material formed of a composite prepared by having a nonwoven fabric formed of all-aromatic polyamide fibers impregnated with a thermosetting epoxy resin. The reference alphanumeric characters 22a and 22b are mask films formed of a plastic film, respectively, measuring 300 mm wide and about 16 xcexcm thick and having a Si-based mold release layer of less than 0.01 xcexcm in thickness disposed on the surface thereof that is in contact with prereg sheet 21. Polyethyleneterephthalate is used as such, for example.
As the method for putting together prereg 21 and mask films 22a and 22b, disclosure is made about a method for continuously attaching by adhesion mask films 22a and 22b on prereg sheet 21 with the resin content thereof melted by the use of a laminating machine. (See the Japanese Patent Application Unexamined Publication No. H7-106766.) The reference numeral 23 is a through hole, which is filled with conductive paste 24 to connect electrically to metal foils 25a and 25b, each measuring 35 xcexcm in thickness, formed of copper and the like and stuck onto both surfaces of prereg sheet 21, respectively.
The conventional manufacturing method of two-sided circuit boards comprises the steps of forming first through hole 23 on prereg sheet 21, on both surfaces of which mask films 22a and 22b are attached by adhesion, respectively, [FIG. 8(a)] at a predetermined position thereof by a laser beam machining method and the like as FIG. 8(b) shows, and then filling conductive paste 24 in through hole 23 as FIG. 8(c) shows. The method for filling conductive paste 24 comprises the steps of placing prereg sheet 21 having through hole 23 formed thereon on a stage of an ordinary printing machine (not shown in FIG. 8) and then filling conductive paste 24 in through hole 23 directly from the top of mask film 22a by shuttling two squeegees formed of urethane rubber and the like to and fro alternately. At this time, mask films 22a and 22b on prereg sheet 21 act as a print mask, respectively, and also serve as means for preventing the surface of prereg sheet 21 from getting contaminated.
A description is further given to the method for filling conductive paste 24 with reference to FIG. 9, FIG. 10 and FIG. 11(a) to FIG. 11(g). A squeegeeing method is used to fill conductive paste 24. However, since mask films 22a and 22b designed specifically for use with prereg sheet 21 are put in place, mask 2, which is formed of stainless steel, measuring about 3 mm in thickness and provided with an opening of an area of 250 mm by 450 mm larger than the effective paste filling area of prereg sheet 21, is mounted on plate framework 1 of a plate for printing 10 for paste filling as FIG. 9 and FIG. 10 show. A slanting area with a slanting angle about 15xc2x0 provided in opening 4 of mask 2 in the squeegees forward moving direction (at the 450 mm side) for the whole purpose of facilitating the passage of the squeegee.
The process of filling conductive paste 24 comprises the steps of setting mask 2 on prereg sheet 21, which is attached by adhesion with mask films 22a and 22b on both surfaces thereof, respectively, formed with through hole 23 thereon and disposed on stage 6 of a printing machine (not shown in FIG. 11) as FIG. 11(a) shows, and lowering the position of only a moving forth squeegee 5a out of both moving forth squeegee 5a and moving back squeegee 5b, which are located above, made movable up and down and right and left, and can be applied with a pressing force, to a predetermined position on mask 2 and having moving forth squeegee 5a moved forward while keeping conductive paste 24 rolling with a pressing force applied thereto. Air is used as the source of a pressing force to be applied to squeegees.
As FIG. 11(b) shows, moving forth squeegee 5a passes the slanting area of mask 2 and reaches the surface of prereg sheet 21. Both moving forth squeegee 5a and moving back squeegee 5b are provided with the ability to move up and down freely according to the positions thereof while maintaining a pressing force to be applied thereto. Thereafter, as FIG. 11(c) shows, moving forth squeegee 5a stops at a predetermined position on mask 2 after travelling on prereg sheet 21 and passing again another slanting area of mask 2, moves upward and then let conductive paste 24 fall freely.
Next, as FIG. 11(d) shows, only moving back squeegee 5b is made to move downward to a predetermined position on mask 2. Then, as FIG. 11(e) to FIG. 11(g) show, by having moving back squeegee 5b moved over mask 2 and prereg sheet 21 in the same manner as moving forth squeegee 5a, the step of filling conductive paste 24 in through hole 23 is completed.
Then, as FIG. 8(d) shows, mask films 22a and 22b are peeled off from both surfaces of prereg sheet 21 and metal foils 25a and 25b formed of copper and the like are superimposed on both surfaces of prereg sheet 21, respectively, as FIG. 8(e) shows. By having a pressing force applied to the foregoing laminate while heat being applied thereto by means of a heat press, not only prereg sheet 21 is compressed in the thickness direction thereof (t2=about 100 xcexcm) but also prereg sheet 21 is joined by adhesion with metal foils 25a and 25b as FIG. 8(f) shows. Meanwhile, metal foils 25a and 25b on the surfaces of prereg sheet 21 are connected electrically with each other via conductive paste 24 filled in through hole 23 formed in prereg sheet 21 at a predetermined position thereof. Then, circuit patterns (not shown in FIG. 8) are formed on metal foils 25a and 25b on the surfaces of prereg sheet 21 by selective etching, thus allowing a two-sided circuit board to be obtained.
However, according to the conventional method for filling a conductive paste as described in above, when the conductive paste is filled in by moving squeegees back and forth, the conductive paste is dropped freely by lifting a moving forth or a moving back squeegee. When this process is repeated to fill in the conductive paste, the squeegee descends on a predetermined position on a mask with some of the paste still remained on the squeegee, thereby allowing the conductive paste to extend to a non-printing surface side of the squeegee (i.e., a complementary angle side).
When the squeegee is moved forward to fill in the conductive paste with some of the conductive paste extended to the non-printing surface side of the squeegee (i.e., the complementary angle side), especially when a conductive paste of low viscosity or a dilatant conductive paste is used, the paste attached to the non-printing surface side of the squeegee (i.e., the complementary angle side) falls on the through hole of the prereg sheet during the movement of the squeegee due to gravity or low viscosity of the conductive paste, resulting in a problem of causing sometimes such an adverse effect on product quality as transferring part of the conductive paste to the mask film side when the mask film is peeled off from the prereg sheet.
In order to deal with the problem as described in above, a plate for printing and a printing-filling in method using the plate for printing of the present invention feature a removal of the conductive paste attached to the non-printing side (i.e., the complementary angle side) of a squeegee, thereby allowing circuit boards with excellent quality to be obtained.